1. Field of the Invention
This invention relates to a control device for a hybrid electric vehicle, and particularly a control device for a hybrid electric vehicle arranged such that a driving force of an electric motor can be transmitted to driving wheels of the vehicle and that a rotary shaft of the electric motor can be coupled with an output shaft of an engine.
2. Description of the Related Art
A so-called parallel hybrid electric vehicle capable of transmitting a driving force of an engine and a driving force of an electric motor to driving wheels of the vehicle has been developed and come into practical use.
In such hybrid electric vehicle, since the output shaft of the engine can be coupled with the rotary shaft of the electric motor, the engine can be started using a driving force of the electric motor that is operated as a motor.
A hybrid electric vehicle arranged such that the engine is started using the driving force of the electric motor in this manner is proposed in Unexamined Japanese Patent Publication No. 2000-64873 (hereinafter referred to as Patent Document 1), for example.
In the case of the hybrid electric vehicle shown in Patent Document 1, when the vehicle which is being driven only by the electric motor with the engine stopped requires a great driving force for a rapid acceleration of the vehicle or the like, the engine is automatically started by the electric motor so that also the engine supplies a driving force. In this hybrid electric vehicle, when the voltage of the battery has decreased to or below a set value, or when a cranking signal from the engine is not detected, the engine cannot be started by the electric motor. Thus, the engine is started by a starter motor in place of the electric motor.
Meanwhile, in order to start traveling of the vehicle on a slope easily, there has been developed and come into practical use a braking force maintaining device for maintaining a braking force on a vehicle at rest on a slope. The braking force maintaining device is arranged to terminate the maintaining of the braking force according to a driver's manipulation for starting the vehicle. An electric vehicle provided with such braking force maintaining device is proposed in Unexamined Japanese Patent Publication No. Hei 9-188233 (hereinafter referred to as Patent Document 2).
The braking force maintaining device shown in Patent Document 2 includes a solenoid valve for supplying a hydraulic pressure to a wheel cylinder for producing a braking force, where the braking force is maintained by activating a solenoid of the solenoid valve, thereby closing the solenoid valve.
When such braking force maintaining device is applied to the hybrid electric vehicle of Patent Document 1, however, the starter motor and the braking force maintaining device share the same electric power source. Thus, there is a possibility that when the starter motor is operated, the power source voltage drops, so that the solenoid valve of the braking force maintaining device cannot be held closed. Consequently, there is a problem that if the solenoid valve is temporality opened due to the operation of the starter motor, the maintaining of the braking force by the braking force maintaining device is accidentally broken off.
In particular, in recent years, vehicles provided with an engine having a so-called idling stop function, namely a function of stopping the engine when the vehicle is at rest, have come into practical use. Thus, if the braking force maintaining device and the idling stop function are applied to the hybrid electric vehicle of Patent Document 1, such problem may happen frequently, since the engine is started/stopped frequently.